1. Field of the Invention
The present invention relates to an apparatus for magnetizing a convergence device for in-line color-picture tubes and to methods of adjusting convergence with such apparatus.
2. Description of the Prior Art
Color-picture tubes commonly have a screen provided with phosphors of three different colors. Each color of phosphor is excited by one of three electron beams emitted by an electron-gun system. Color-picture tubes in which the three electron guns of the electron-gun system are arranged in one plane are called "in-line color-picture tubes". In such in-line tubes, a so-called shadow mask is mounted between the electron-gun system and the luminescent screen. The electron beams pass through slit openings of the mask and strike the phosphor areas. For proper operation of the in-line color-picture tube it is necessary that all three electron beams intersect in each of the mask slits. To achieve this intersection, the electron beams are deflected in the electron-gun system by static magnetic fields. The adjustment of these magnetic fields is called "convergence adjustment".
The principle of such a convergence adjustment is described in the journal "Funkschau" 1976, No. 5, pages 59 and 60. In the principle described there, the two outer ones of the coplanar electron beams are first converged at the same point. To accomplish this, the two outer beams are moved synchronously with or relative to each other in the horizontal or vertical direction by means of four-pole fields. By means of six-pole fields, the two outer converged electron beams are movable together in relation to the central beam in the horizontal and vertical directions. Such adjusting movements will hereinafter be referred to as "deflection movements". To be able to perform other deflection movements, one of the outer electron beams is adjustable in the horizontal and vertical directions essentially independently of the central beam and the other outer beam. In this manner, each of the two outer beams is converged individually on the central beam.
In the above-mentioned "Funkschau" article, convergence adjustments are made by rotating premagnetized magnetic rings located around the neck of the tube above the electron-gun system. However, it is already known from German Pat. No. 961,735 to deflect an electron beam by placing a permanent magnet material inside the tube neck and selectively magnetizing or remagnetizing this material from outside.
It is customary to use hard magnetic materials for convergence adjustments. These materials are mounted inside the tube neck, possibly directly to the electron-gun system, and are magnetized or remagnetized from outside by means of a magnetizing apparatus. The present invention relates to magnetizing apparatus of the latter kind.
DE-OS No. 28 28 710 discloses apparatus and methods suitable for magnetizing and remagnetizing hard magnetic materials for making convergence adjustments. FIGS. 11 and 12 show two apparatus in each of which eight coils are disposed radially around a tube neck. The coils are wired up so that the two-, four-, and six-pole fields necessary for adjustment can be achieved. It has turned out that only small deflections of the electron beams are possible with such apparatus. In DE-OS No. 28 32 666, FIG. 2 shows a magnetizing apparatus in which the magnet coils are mounted in two planes and one behind the other in direction of the beams. This permits the number of coils to be increased considerably, which results in stronger magnetic fields and, hence, greater deflection. However, all deflection fields for electron beams change not only the direction of the beam but also its shape. For the shapes of the electron beams, deflection in two different planes has proved disadvantageous.
The electron beams are deflected by a convergence device to adjust not only convergence but color purity. Purity adjustment necessitates moving all three electron beams jointly in the horizontal direction. In the magnetizing apparatus described above, the two-pole fields necessary therefor are obtained by suitably connecting the radial, electrically excitable magnet coils. DE-OS No. 28 32 667 describes a different magnetizing apparatus in which purity is adjusted by means of conductors around the tube neck. The apparatus shown in FIG. 6 of that application uses two radial magnet coils located in two planes, and conductors arranged in a third plane in front of the two other planes. There, too, the problem of beam deformation arises as a result of the deflection in different planes.